The design of the top tie plate or the top tie plates of the fuel assembly is very important for the performance of the fuel assembly. When coolant flows upwardly through the fuel assembly, it is important that the coolant be subjected to as low a pressure drop as possible. A low pressure drop in the fuel assembly is favorable for the stability properties of the fuel assembly. A low pressure drop also results in an increase of the capacity in the pumps which provide the fuel assembly with coolant, as well as an increase of the flexibility during operation of the reactor. Further, it is important that the top tie plate or the top tie plates be designed for lifting a bundle or four sub-bundles out of the fuel assembly.
FIG. 1 shows a prior art fuel assembly 1 for a boiling water reactor, and FIG. 3 shows in a view from above a prior art top tie plate 5a for a sub-bundle. Extra long fuel rods 3a, which partially extend above the top tie plate 5a, are adapted to run through two of the openings in the top tie plate 5a. The two extra long fuel rods 3a are fixed to the bottom tie plate 6 and are each provided, at the upper side of the top tie plate 5a, with a nut 8. The other rods 3 rest on the bottom tie plate 6 and make contact with the lower side of the top tie plate 5a by means of a helical spring 9 arranged around the respective end plugs 10 of the rods 3. The helical springs press the top tie plate 5a against the nuts 8, whereby the nuts 8 limit the maximum distance between the top and bottom tie plates 5a, 6. The two extra long fuel rods 3a are arranged in a tensile-force-transmitting connection between the top and bottom tie plates 5a, 6 while at the same time they limit the maximum distance between the top and bottom tie plates 5a, 6.
The top tie plate 5a is designed to receive a handling tool in a respective handling opening 11 located adjacent to the openings for the extra long rods 3a. The handling tool is passed through the top tie plate 5a by way of the handling openings 11, whereupon they are adapted to engage bars 5c at the respective handling opening 11 and thereafter to lift the sub-bundle out of the fuel assembly. The lifting movement means that a large part of the load is transferred from the bottom tie plate 6 to the supporting extra long rods 3a and to the top tie plate 5a via the nuts 8 which are threaded on the plugs 10 of the supporting extra long rods 3a above the top tie plate 5a and further to the handling tool. For the top tie plate 5a to manage this lifting movement, it must be designed from the strength point of view to manage the load of the whole sub-bundle. This means that the amount of material becomes large and hence that a certain pressure drops arises over the top tie plate 5a. Further, the strength of the top tie plate 5a imposes a limit to the load to which it may be subjected.
One problem with the embodiment described is that the supporting extra long fuel rods may have grown to different degrees upon irradiation during operation of the reactor, whereby the axial position of the nuts become different for the supporting rods, which means that the load may have an uneven distribution so that the whole load must be lifted in one rod. When lifting in this top tie plate, tools are required which can be guided through the handling opening and then engage the above-mentioned bars from the lower side of the top tie plate.
Differential rod growth may imply that it may be difficult to reach under the bars with the remote-controlled tool since the other fuel rods arranged with helical springs against the lower side of the top tie plate may have grown in the axial direction and approached the top tie plate and hence considerably reduced the space for the handling tool under the bars.
It is also known, in a fuel assembly comprising a bundle with a water tube arranged centrally therein, to arrange the water tube in a tensile-force-transmitting connection between the top and bottom tie plates and to lift out the bundle by lifting in the water tube. Thus, the water tube has two functions, namely, to fix the top tie plate and to transfer part of the load to the bottom tie plate.
The object of the invention is to provide a fuel assembly with a top tie plate which constitutes a low flow resistance, which does not impose any limit to the lifting force required to lift out a bundle or one or more sub-bundles. Further, the object of the invention is to achieve a fuel assembly in which the tensile-force-transmitting element or elements during handling of the bundle does/do not include the top tie plate.